Make AbstractNode-derived docs more specific on RenderObject et al.

[gnprice]

These methods and/or their docs were recently copied (in #128467 and #128973) from their classes' former shared base class AbstractNode. Their wording was fittingly abstract there, but that abstraction is a bit puzzling for a reader finding them on these more concrete classes and not aware of the AbstractNode history. So make them more concrete, in similar terms to the other methods around them.

Also copy some useful points between corresponding methods on different classes (like that the parent of the root is null), and try to clean up the prose on [RenderObject.depth].

We focus on the more outward-facing parts of the API, letting methods like redepthChildren continue to talk generically about "nodes".

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[gnprice]

This is a sort of followup to #130688, which is about some of the same docs. Sent as a separate PR because I think that will make both sets of changes simpler to review, because the actual content of the changes is largely unrelated.

[Hixie]

Thank you for improving the documentation!